A known pressure sensor, as shown in U.S. Pat. No. 4,716,492, comprises a capacitive pressure transducer having a thin, relatively flexible ceramic diaphragm mounted in closely spaced, sealed, overlying relation to a rigid ceramic substrate and having metal coatings deposited on respective opposing surfaces of the diaphragm and substrate to serve as source and detect capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Electrically conductive traces extend out to pins received in bores formed through the substrate located between the capacitor plates and the outer periphery of the diaphragm and substrate which are connected to an electronic conditioning module attached to the transducer. The diaphragm flexes in response to pressure and causes the source and detect plates to move closer together thereby increasing the capacitance between the plates which is measured by the electronic conditioning module. An annular guard ring of electrically conductive material is printed on the substrate around the detect plate and electrically held at the same voltage as the detect plate. Typically, the guard ring has an inner diameter slightly less than the diameter of the circular source plate. This ring serves as a guard to reduce the electrical field intensity between the source and detect plates at the edges of the detect plate. These fringe electric fields are undesirable because they cause a non-linear pressure transducer output. The electronic conditioning module is designed to measure the capacitance between the source and detect plates only and is insensitive to capacitance between the source plate and the guard, between the detect plate and the guard or between either the source plate or the detect plate and the housing of the sensor.
When used with polar or conductive fluids it has been found that the transducer output shifts by up to 1% full scale or more. Since capacitance is dependent upon both the electric field between the capacitor plates and any dielectric material within the electric field, it is believed that the output error is caused by electric fields between the source and detect plates traveling through the diaphragm and into the working fluid so that when the dielectric coefficient of the working fluid changes the transducer capacitance also changes. In view of the fact that the pressure transducers are used to monitor the pressure of many fluids including those which are polar or conductive, such as water, this error is undesirable.
One proposed solution is to place a thin discrete metal shield on the diaphragm connected to the transducer housing through a compliant, electrically conductive material, such as brass wool. The conductive shield covering the diaphragm and connected to the housing would act as a guard for the entire transducer, that is, the electric fields would not pass through the conductive shield and, therefore, could not be affected by material on the opposite side of the shield. However, this approach is unsatisfactory for several reasons including the possibility of pieces of the wool deteriorating and contaminating the fluid, the effect of pressure from the compliant wool on the transducer output, the durability of the metal shield and questions of compatibility with various working fluids, possible hysteresis due to the metal shield and the question of long term durability of the electrical contact between the shield and the housing.
Another proposed solution is the use of a metal shield printed on the surface of the diaphragm using, for example, the same material, e.g., gold, which is used for the electrically conductive capacitor plates and traces. The printed shield would be connected to the housing using a compliant mechanism such as a washer and a wave spring. However, this approach involves the addition of components which add to the cost of the sensor. It would be desirable to provide a sensor in which the output error is minimized or eliminated without appreciably impacting the cost of the sensor.